Transparent, biaxially oriented polyester films which feature improved barrier properties are disclosed by the prior art. In most cases, the films obtain their improved barrier properties offline after the production by a further processing step. Examples thereof are extrusion coating, coating or lamination with barrier materials, coating under reduced pressure with metals or with ceramic substances, or plasma polymerization in conjunction with vacuum coating.
An exception is the process described in detail in WO 99/62694, in which a multilayer, coextruded polyester film which comprises at least one layer of EVOH (ethylene-vinyl alcohol) is simultaneously biaxially stretched. The film features good mechanical properties, but in particular good barrier properties against the passage of oxygen. The best value reported in the document for the achievable oxygen transmission rate (OTR) is 5 cm3/(m2·bar·d). A disadvantage of the process is that regrind obtained in the course of production cannot be fed back to the process without the film losing its good optical properties.
A further exception is the biaxially oriented film which is described in JP 2001-347592 and consists of a mixture of polyethylene terephthalate and poly(m-xyleneadipamide) (MXD6). The proportion of poly(m-xyleneadipamide) (MXD6) in the film is between 10 and 40% by weight, and the corresponding proportion of polyethylene terephthalate is between 60 and 90% by weight. The film is simultaneously biaxially stretched. It features good mechanical properties, high thermal stability, but in particular a good barrier with respect to the permeation of oxygen. The film achieves an OTR which is less than 30 cm3/(m2·bar·d). In addition, the film features low opacity. The document reports a value for the opacity which is below 15%. A disadvantage of the film is its two rough surfaces which result from the mixing incompatibility of the two raw materials (polyethylene terephthalate and poly(m-xyleneadipamide) (MXD6)). The high roughness values result in the film losing its originally high gloss and obtaining a matt appearance which is undesired for many applications in the packaging industry. A further disadvantage of the film is that it cannot be printed, metalized or coated as efficiently as films which consist, for example, only of polyethylene terephthalate. As a result of the high roughness of the two film surfaces, the thickness distribution of additionally applied layers (printing inks, metals, varnish, ceramic materials) is inadequate. This likewise worsens the physical properties of these layers.
Likewise an exception is the film of EP-B-0 675 158 which is an oriented composite film based on polyester and having improved barrier properties toward gases. The film is coated on at least one of the two sides with a layer, having a thickness of 0.3 μm or less, of polyvinyl alcohol which has a number-average degree of polymerization of 350 and more, and the average roughness Rz of the side of the base film to be coated is 0.30 μm or less, and this side features a certain distribution of elevations on the film surface. The composite film has an OTR which is less than 3 cm3/(m2·bar·d). A disadvantage of this composite film is its low resistance, for example, toward moisture. On contact with water or steam, the adhesion of the barrier coating of polyvinyl alcohol to the polyester film is lost, with the effect that the barrier coating can be washed off the polyester film.